1. Field of the Invention
The present invention concerns a tunable diode laser system with external resonator in Littrow or Littman configuration. The system includes an optical lattice on which the light beam from a laser diode is diffracted, a support element to hold the lattice or to hold a mirror, which reflects the light diffracted by the lattice, and an actuator to change the position of the lattice or the mirror.
2. The Prior Art
The spectral bandwidth of the laser emission can be substantially reduced by the back-coupling of the light which is emitted by a laser diode of a diode laser with external resonator. An external resonator, furthermore, makes it possible to tune the laser wavelength. From the prior art, configurations are known with back-coupling via an optical lattice, on which the light generated by means of the laser diode is diffracted. The lattice can be arranged at the front end of the laser diode, i.e., in the useful beam, or at the back end of the laser diode, in which case the back-coupling occurs via the end facet of the laser diode.
For diode lasers in the so-called Littrow configuration, the light emitted by the laser diode is collimated by means of a collimation optics and diffracted on the optical lattice. The light of the zeroth diffraction order is taken out as the useful beam. The light of the first diffraction order is reflected back into the laser diode. In this way, the diffraction lattice and the end facet of the laser diode form a resonator.
The tuning of the wavelength for a diode laser in Littrow configuration occurs by rotating the optical lattice. This rotation produces a change in the angle of incidence of the laser beam on the lattice and, consequently, a variation in the wavelength of the resonantly back-coupled first diffraction order.
In order to tune the diode laser system free of mode hops, one must vary the length of the optical resonator at the same time as the angle of rotation of the diffraction lattice is changed, so that always the same laser mode prevails in the resonator. A coordinated changing of angle and length is realized precisely when the diffraction lattice is turned about a pivot center defined by the intersection of the planes of the surface of the optical lattice and the end facet of the laser diode.
In practice, such an axis of rotation unfortunately cannot be accurately realized, or it requires great expense to do so. Furthermore, the necessarily long turning arm of the diffraction lattice leads to a very large structural shape. As a result, the disadvantages occur that the system reacts sensitively to low frequency mechanical vibrations and can be thermally controlled only with difficulty. The long turning arm, furthermore, is a disadvantage, because the adjustment can easily be disturbed by the forces produced during transport.
Furthermore, diode laser systems are known in the so-called Littman configuration. In these systems, the laser beam emitted by a laser diode is likewise diffracted on an optical lattice, but the light of the first diffraction order impinges on a mirror, which reflects the light back onto the diffraction lattice. The mirror forms with the end facet of the laser diode an optical resonator. To tune the wavelength, the mirror is turned. This turning changes the angle between the diffracted beam of first order and the normal to the optical lattice, which dictates the resonance condition. For a tuning of the wavelength free of mode hops, the mirror in a diode laser system in the Littman configuration must be turned about a pivot center which is defined by the line of intersection of three planes. These planes are the plane of the end facet of the laser diode, the plane of the surface of the diffraction lattice, and the plane of the surface of the mirror. The accurate realization of this center of rotation requires, in practice, a very complicated mechanism. The fabrication and maintenance of such laser systems is accordingly costly and time-intensive. A flawless position of the center of rotation significantly limits the tuning range free of mode hops.
A tunable diode laser system of the above indicated kind, in which actuators are provided for the changing of the position of the lattice, is known from the prior art (J. Hult et al.: “Wide-bandwidth mode-hop-free tuning of extended-cavity GaN diode lasers”, Applied Optics, Jun. 20, 2005, Vol. 44, No. 18, p. 3675-3679). In the known system, a support element is provided to support the diffraction lattice. The support element includes two piezo-actuators, which bring about a turning of the diffraction lattice and a synchronous changing of the length of the optical resonator. The turning occurs about a (virtual) center of rotation, which is defined by the actuation of the piezo-actuators. The disadvantage in this system is that it requires two separate piezo-actuators. The correct actuation of the piezo-actuators to realize a mode-hop-free tuning is complicated, which makes necessary an equally costly electronics. Thus, the system is elaborate and costly to produce.